How to test navigation via Navigator in Flutter

While what Danny said is correct and works, you can also create a mocked NavigatorObserver to avoid any extra boilerplate:

import 'package:mockito/mockito.dart';

class MockNavigatorObserver extends Mock implements NavigatorObserver {}

That would translate to your test case as follows:

void main() {
  testWidgets('Button is present and triggers navigation after tapped',
      (WidgetTester tester) async {
    final mockObserver = MockNavigatorObserver();
    await tester.pumpWidget(
      MaterialApp(
        home: MyScreen(),
        navigatorObservers: [mockObserver],
      ),
    );

    expect(find.byType(RaisedButton), findsOneWidget);
    await tester.tap(find.byType(RaisedButton));
    await tester.pumpAndSettle();

    /// Verify that a push event happened
    verify(mockObserver.didPush(any, any));

    /// You'd also want to be sure that your page is now
    /// present in the screen.
    expect(find.byType(DetailsPage), findsOneWidget);
  });
}

I wrote an in-depth article about this on my blog, which you can find here.